LJMU Research Onlineresearchonline.ljmu.ac.uk/5303/1/1609.04153v1.pdf · Astronomy & Astrophysics manuscript no. 29272 c ESO 2016 September 15, 2016 The Gaia mission Gaia Collaboration,

Brown, AGA, Vallenari, A, Prusti, T, de Bruijne, JHJ, Mignard, F, Drimmel, R, Babusiaux, C, Bailer-Jones, CAL, Bastian, U, Biermann, M, Evans, DW, Eyer, L, Jansen, F, Jordi, C, Katz, D, Klioner, SA, Lammers, U, Lindegren, L, Luri, X, O'Mullane, W, Panem, C, Pourbaix, D, Randich, S, Sartoretti, P, Siddiqui, HI, Soubiran, C, Valette, V, van Leeuwen, F, Walton, NA, Aerts, C, Arenou, F, Cropper, M, Hog, E, Lattanzi, MG, Grebel, EK, Holland, AD, Huc, C, Passot, X, Perryman, M, Bramante, L, Cacciari, C, Castaneda, J, Chaoul, L, Cheek, N, De Angeli, F, Fabricius, C, Guerra, R, Hernandez, J, Jean-Antoine-Piccolo, A, Masana, E, Messineo, R, Mowlavi, N, Nienartowicz, K, Ordonez-Blanco, D, Panuzzo, P, Portell, J, Richards, PJ, Riello, M, Seabroke, GM, Tanga, P, Thevenin, F, Torra, J, Els, SG, Gracia-Abril, G, Comoretto, G, Garcia-Reinaldos, M, Lock, T, Mercier, E, Altmann, M, Andrae, R, Astraatmadja, TL, Bellas-Velidis, I, Benson, K, Berthier, J, Blomme, R, Busso, G, Carry, B, Cellino, A, Clementini, G, Cowell, S, Creevey, O, Cuypers, J, Davidson, M, De Ridder, J, de Torres, A, Delchambre, L, Dell'Oro, A, Ducourant, C, Fremat, Y, Garcia-Torres, M, Gosset, E, Halbwachs, J-L, Hambly, NC, Harrison, DL, Hauser, M, Hestroffer, D, Hodgkin, ST, Huckle, HE, Hutton, A, Jasniewicz, G, Jordan, S, Kontizas, M, Korn, AJ, Lanzafame, AC, Manteiga, M, Moitinho, A, Muinonen, K, Osinde, J, Pancino, E, Pauwels, T, Petit, J-M, Recio-Blanco, A, Robin, AC, Sarro, LM, Siopis, C, Smith, M, Smith, KW, Sozzetti, A, Thuillot, W, van Reeven, W, Viala, Y, Abbas, U, Aramburu, AA, Accart, S, Aguado, JJ, Allan, PM, Allasia, W, Altavilla, G, Alvarez, MA, Alves, J, Anderson, RI, Andrei, AH, Varela, EA, Antiche, E, Antoja, T, Anton, S, Arcay, B, Bach, N, Baker, SG, Balaguer-Nunez, L, Barache, C, Barata, C, Barbier, A, Barblan, F, Barrado y Navascues, D, Barros, M, Barstow, MA, Becciani, U, Bellazzini, M, Garcia, AB, Belokurov, V, Bendjoya, P, Berihuete, A, Bianchi, L, Bienayme, O, Billebaud, F, Blagorodnova, N, Blanco-Cuaresma, S, Boch, T, Bombrun, A, Borrachero, R, Bouquillon, S, Bourda, G, Bouy, H, Bragaglia, A, Breddels, MA, Brouillet, N, Bruesemeister, T, Bucciarelli, B, Burgess, P, Burgon, R, Burlacu, A, Busonero, D, Buzzi, R, Caffau, E, Cambras, J, Campbell, H, Cancelliere, R, Cantat-Gaudin, T, Carlucci, T, Carrasco, JM, Castellani, M, Charlot, P, Charnas, J, Chiavassa, A, Clotet, M, Cocozza, G, Collins, RS, Costigan, G, Crifo, F, Cross, NJG, Crosta, M, Crowley, C, Dafonte, C, Damerdji, Y, Dapergolas, A, David, P, David, M, De Cat, P, de Felice, F, de laverny, P, De Luise, F, De March, R, de Martino, D, de

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Souza, R, Debosscher, J, del Pozo, E, Delbo, M, Delgado, A, Delgado, HE, Di Matteo, P, Diakite, S, Distefano, E, Dolding, C, Dos Anjos, S, Drazinos, P, Duran, J, Dzigan, Y, Edvardsson, B, Enke, H, Evans, NW, Bontemps, GE, Fabre, C, Fabrizio, M, Faigler, S, Falcao, AJ, Casas, MF, Federici, L, Fedorets, G, Fernandez-Hernandez, J, Fernique, P, Fienga, A, Figueras, F, Filippi, F, Findeisen, K, Fonti, A, Fouesneau, M, Fraile, E, Fraser, M, Fuchs, J, Gai, M, Galleti, S, Galluccio, L, Garabato, D, Garcia-Sedano, F, Garofalo, A, Garralda, N, Gavras, P, Gerssen, J, Geyer, R, Gilmore, G, Girona, S, Giuffrida, G, Gomes, M, Gonzalez-Marcos, A, Gonzalez-Nunez, J, Gonzalez-Vidal, JJ, Granvik, M, Guerrier, A, Guillout, P, Guiraud, J, Gurpide, A, Gutierrez-Sanchez, R, Guy, LP, Haigron, R, Hatzidimitriou, D, Haywood, M, Heiter, U, Helmi, A, Hobbs, D, Hofmann, W, Holl, B, Holland, G, Hunt, JAS, Hypki, A, Icardi, V, Irwin, M, de Fombelle, GJ, Jofre, P, Jonker, PG, Jorissen, A, Julbe, F, Karampelas, A, Kochoska, A, Kohley, R, Kolenberg, K, Kontizas, E, Koposov, SE, Kordopatis, G, Koubsky, P, Krone-Martins, A, Kudryashova, M, Kull, I, Bachchan, RK, Lacoste-Seris, F, Lanza, AF, Lavigne, J-B, Le Poncin-Lafitte, C, Lebreton, Y, Lebzelter, T, Leccia, S, Leclerc, N, Lecoeur-Taibi, I, Lemaitre, V, Lenhardt, H, Leroux, F, Liao, S, Licata, E, Lindstrom, HEP, Lister, TA, Livanou, E, Lobel, A, Loeffler, W, Lopez, M, Lorenz, D, MacDonald, I, Fernandes, TM, Managau, S, Mann, RG, Mantelet, G, Marchal, O, Marchant, JM, Marconi, M, Marinoni, S, Marrese, PM, Marschalko, G, Marshall, DJ, Martin-Fleitas, JM, Martino, M, Mary, N, Matijevic, G, Mazeh, T, McMillan, PJ, Messina, S, Michalik, D, Millar, NR, Miranda, BMH, Molina, D, Molinaro, R, Molinaro, M, Molnar, L, Moniez, M, Montegriffo, P, Mor, R, Mora, A, Morbidelli, R, Morel, T, Morgenthaler, S, Morris, D, Mulone, AF, Muraveva, T, Musella, I, Narbonne, J, Nelemans, G, Nicastro, L, Noval, L, Ordenovic, C, Ordieres-Mere, J, Osborne, P, Pagani, C, Pagano, I, Pailler, F, Palacin, H, Palaversa, L, Parsons, P, Pecoraro, M, Pedrosa, R, Pentikainen, H, Pichon, B, Piersimoni, AM, Pincau, F-X, Plachy, E, Plum, G, Poujoulet, E, Prsa, A, Pulone, L, Ragaini, S, Rago, S, Rambaux, N, Ramos-Lerate, M, Ranalli, P, Rauw, G, Read, A, Regibo, S, Reyle, C, Ribeiro, RA, Rimoldini, L, Ripepi, V, Riva, A, Rixon, G, Roelens, M, Romero-Gomez, M, Rowell, N, Royer, F, Ruiz-Dern, L, Sadowski, G, Selles, TS, Sahlmann, J, Salgado, J, Salguero, E, Sarasso, M, Savietto, H, Schultheis, M, Sciacca, E, Segol, M, Segovia, JC, Segransan, D, Shih, I-C, Smareglia, R, Smart, RL, Solano, E, Solitro, F, Sordo, R, Nieto, SS, Souchay, J, Spagna, A, Spoto, F, Stampa, U, Steele, IA, Steidelmueller, H, Stephenson, CA, Stoev, H, Suess, FF, Suveges, M, Surdej, J, Szabados, L, Szegedi-Elek, E, Tapiador, D, Taris, F, Tauran, G, Taylor, MB, Teixeira, R, Terrett, D, Tingley, B, Trager, SC, Turon, C, Ulla, A, Utrilla, E, Valentini, G, van Elteren, A, Van Hemelryck, E, van Leeuwen, M, Varadi, M, Vecchiato, A, Veljanoski, J, Via, T, Vicente, D, Vogt, S, Voss, H, Votruba, V, Voutsinas, S, Walmsley, G, Weiler, M, Weingrill, K, Wevers, T, Wyrzykowski, L, Yoldas, A, Zerjal, M, Zucker, S, Zurbach, C, Zwitter, T, Alecu, A, Allen, M, Prieto, CA, Amorim, A, Anglada-Escude, G, Arsenijevic, V, Azaz, S,


Balm, P, Beck, M, Bernstein, H-H, Bigot, L, Bijaoui, A, Blasco, C, Bonfigli, M, Bono, G, Boudreault, S, Bressan, A, Brown, S, Brunet, P-M, Bunclark, P, Buonanno, R, Butkevich, AG, Carret, C, Carrion, C, Chemin, L, Chereau, F, Corcione, L, Darmigny, E, de Boer, KS, de Teodoro, P, de Zeeuw, PT, Delle Luche, C, Domingues, CD, Dubath, P, Fodor, F, Frezouls, B, Fries, A, Fustes, D, Fyfe, D, Gallardo, E, Gallegos, J, Gardiol, D, Gebran, M, Gomboc, A, Gomez, A, Grux, E, Gueguen, A, Heyrovsky, A, Hoar, J, Iannicola, G, Parache, YI, Janotto, A-M, Joliet, E, Jonckheere, A, Keil, R, Kim, D-W, Klagyivik, P, Klar, J, Knude, J, Kochukhov, O, Kolka, I, Kos, J, Kutka, A, Lainey, V, LeBouquin, D, Liu, C, Loreggia, D, Makarov, VV, Marseille, MG, Martayan, C, Martinez-Rubi, O, Massart, B, Meynadier, F, Mignot, S, Munari, U, Nguyen, A-T, Nordlander, T, Ocvirk, P, O'Flaherty, KS, Sanz, AO, Ortiz, P, Osorio, J, Oszkiewicz, D, Ouzounis, A, Palmer, M, Park, P, Pasquato, E, Peltzer, C, Peralta, J, Peturaud, F, Pieniluoma, T, Pigozzi, E, Poels, J, Prat, G, Prod'homme, T, Raison, F, Rebordao, JM, Risquez, D, Rocca-Volmerange, B, Rosen, S, Ruiz-Fuertes, MI, Russo, F, Sembay, S, Vizcaino, IS, Short, A, Siebert, A, Silva, H, Sinachopoulos, D, Slezak, E, Soffel, M, Sosnowska, D, Straizys, V, ter Linden, M, Terrell, D, Theil, S, Tiede, C, Troisi, L, Tsalmantza, P, Tur, D, Vaccari, M, Vachier, F, Valles, P, Van Hamme, W, Veltz, L, Virtanen, J, Wallut, J-M, Wichmann, R, Wilkinson, MI, Ziaeepour, H and Zschocke, S

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Brown, AGA, Vallenari, A, Prusti, T, de Bruijne, JHJ, Mignard, F, Drimmel, R, Babusiaux, C, Bailer-Jones, CAL, Bastian, U, Biermann, M, Evans, DW, Eyer, L, Jansen, F, Jordi, C, Katz, D, Klioner, SA, Lammers, U, Lindegren, L, Luri, X, O'Mullane, W, Panem, C, Pourbaix, D, Randich, S, Sartoretti, P,


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Astronomy & Astrophysics manuscript no. 29272 c©ESO 2016September 15, 2016

The Gaia missionGaia Collaboration, T. Prusti1, J.H.J. de Bruijne1, A.G.A. Brown2, A. Vallenari3, C. Babusiaux4, C.A.L.

Bailer-Jones5, U. Bastian6, M. Biermann6, D.W. Evans7, L. Eyer8, F. Jansen9, C. Jordi10, S.A. Klioner11, U.Lammers12, L. Lindegren13, X. Luri10, F. Mignard14, D.J. Milligan15, C. Panem16, V. Poinsignon17, D.

Pourbaix18, 19, S. Randich20, G. Sarri21, P. Sartoretti4, H.I. Siddiqui22, C. Soubiran23, V. Valette16, F. van Leeuwen7,N.A. Walton7, C. Aerts24, 25, F. Arenou4, M. Cropper26, R. Drimmel27, E. Høg28, D. Katz4, M.G. Lattanzi27, W.O’Mullane12, E.K. Grebel6, A.D. Holland29, C. Huc16, X. Passot16, L. Bramante30, C. Cacciari31, J. Castañeda10,

L. Chaoul16, N. Cheek32, F. De Angeli7, C. Fabricius10, R. Guerra12, J. Hernández12, A. Jean-Antoine-Piccolo16, E.Masana10, R. Messineo30, N. Mowlavi8, K. Nienartowicz33, D. Ordóñez-Blanco33, P. Panuzzo4, J. Portell10, P.J.

Richards34, M. Riello7, G.M. Seabroke26, P. Tanga14, F. Thévenin14, J. Torra10, S.G. Els35, 6, G. Gracia-Abril35, 10,G. Comoretto22, M. Garcia-Reinaldos12, T. Lock12, E. Mercier35, 6, M. Altmann6, 36, R. Andrae5, T.L.

Astraatmadja5, I. Bellas-Velidis37, K. Benson26, J. Berthier38, R. Blomme39, G. Busso7, B. Carry14, 38, A.Cellino27, G. Clementini31, S. Cowell7, O. Creevey14, 40, J. Cuypers39, M. Davidson41, J. De Ridder24, A. de

Torres42, L. Delchambre43, A. Dell’Oro20, C. Ducourant23, Y. Frémat39, M. García-Torres44, E. Gosset43, 19, J.-L.Halbwachs45, N.C. Hambly41, D.L. Harrison7, 46, M. Hauser6, D. Hestroffer38, S.T. Hodgkin7, H.E. Huckle26, A.

Hutton47, G. Jasniewicz48, S. Jordan6, M. Kontizas49, A.J. Korn50, A.C. Lanzafame51, 52, M. Manteiga53, A.Moitinho54, K. Muinonen55, 56, J. Osinde57, E. Pancino20, 58, T. Pauwels39, J.-M. Petit59, A. Recio-Blanco14, A.C.Robin59, L.M. Sarro60, C. Siopis18, M. Smith26, K.W. Smith5, A. Sozzetti27, W. Thuillot38, W. van Reeven47, Y.Viala4, U. Abbas27, A. Abreu Aramburu61, S. Accart62, J.J. Aguado60, P.M. Allan34, W. Allasia63, G. Altavilla31,

M.A. Álvarez53, J. Alves64, R.I. Anderson65, 8, A.H. Andrei66, 67, 36, E. Anglada Varela57, 32, E. Antiche10, T.Antoja1, S. Antón68, 69, B. Arcay53, A. Atzei21, L. Ayache70, N. Bach47, S.G. Baker26, L. Balaguer-Núñez10, C.Barache36, C. Barata54, A. Barbier62, F. Barblan8, M. Baroni21, D. Barrado y Navascués71, M. Barros54, M.A.Barstow72, U. Becciani52, M. Bellazzini31, G. Bellei73, A. Bello García74, V. Belokurov7, P. Bendjoya14, A.

Berihuete75, L. Bianchi63, O. Bienaymé45, F. Billebaud23, N. Blagorodnova7, S. Blanco-Cuaresma8, 23, T. Boch45,A. Bombrun42, R. Borrachero10, S. Bouquillon36, G. Bourda23, H. Bouy71, A. Bragaglia31, M.A. Breddels76, N.

Brouillet23, T. Brüsemeister6, B. Bucciarelli27, F. Budnik15, P. Burgess7, R. Burgon29, A. Burlacu16, D.Busonero27, R. Buzzi27, E. Caffau4, J. Cambras77, H. Campbell7, R. Cancelliere78, T. Cantat-Gaudin3, T.

Carlucci36, J.M. Carrasco10, M. Castellani79, P. Charlot23, J. Charnas33, P. Charvet17, F. Chassat17, A. Chiavassa14,M. Clotet10, G. Cocozza31, R.S. Collins41, P. Collins15, G. Costigan2, F. Crifo4, N.J.G. Cross41, M. Crosta27, C.

Crowley42, C. Dafonte53, Y. Damerdji43, 80, A. Dapergolas37, P. David38, M. David81, P. De Cat39, F. de Felice82, P.de Laverny14, F. De Luise83, R. De March30, D. de Martino84, R. de Souza85, J. Debosscher24, E. del Pozo47, M.

Delbo14, A. Delgado7, H.E. Delgado60, F. di Marco86, P. Di Matteo4, S. Diakite59, E. Distefano52, C. Dolding26, S.Dos Anjos85, P. Drazinos49, J. Durán57, Y. Dzigan87, 88, E. Ecale17, B. Edvardsson50, H. Enke89, M. Erdmann21, D.

Escolar21, M. Espina15, N.W. Evans7, G. Eynard Bontemps62, C. Fabre90, M. Fabrizio58, 83, S. Faigler91, A.J.Falcão92, M. Farràs Casas10, F. Faye17, L. Federici31, G. Fedorets55, J. Fernández-Hernández32, P. Fernique45, A.

Fienga93, F. Figueras10, F. Filippi30, K. Findeisen4, A. Fonti30, M. Fouesneau5, E. Fraile94, M. Fraser7, J. Fuchs95,R. Furnell21, M. Gai27, S. Galleti31, L. Galluccio14, D. Garabato53, F. García-Sedano60, P. Garé21, A. Garofalo31,N. Garralda10, P. Gavras4, 37, 49, J. Gerssen89, R. Geyer11, G. Gilmore7, S. Girona96, G. Giuffrida58, M. Gomes54,A. González-Marcos97, J. González-Núñez32, 98, J.J. González-Vidal10, M. Granvik55, A. Guerrier62, P. Guillout45,J. Guiraud16, A. Gúrpide10, R. Gutiérrez-Sánchez22, L.P. Guy33, R. Haigron4, D. Hatzidimitriou49, M. Haywood4,U. Heiter50, A. Helmi76, D. Hobbs13, W. Hofmann6, B. Holl8, G. Holland7, J.A.S. Hunt26, A. Hypki2, V. Icardi30,M. Irwin7, G. Jevardat de Fombelle33, P. Jofré7, 23, P.G. Jonker99, 25, A. Jorissen18, F. Julbe10, A. Karampelas49, 37,

A. Kochoska100, R. Kohley12, K. Kolenberg101, 24, 102, E. Kontizas37, S.E. Koposov7, G. Kordopatis89, 14, P.Koubsky95, A. Kowalczyk15, A. Krone-Martins54, M. Kudryashova38, I. Kull91, R.K. Bachchan13, F.

Lacoste-Seris62, A.F. Lanza52, J.-B. Lavigne62, C. Le Poncin-Lafitte36, Y. Lebreton4, 103, T. Lebzelter64, S. Leccia84,N. Leclerc4, I. Lecoeur-Taibi33, V. Lemaitre62, H. Lenhardt6, F. Leroux62, S. Liao27, 104, E. Licata63, H.E.P.Lindstrøm28, 105, T.A. Lister106, E. Livanou49, A. Lobel39, W. Löffler6, M. López71, A. Lopez-Lozano107, D.

Lorenz64, T. Loureiro15, I. MacDonald41, T. Magalhães Fernandes92, S. Managau62, R.G. Mann41, G. Mantelet6, O.Marchal4, J.M. Marchant108, M. Marconi84, J. Marie109, S. Marinoni79, 58, P.M. Marrese79, 58, G. Marschalkó110, 111,

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D.J. Marshall112, J.M. Martín-Fleitas47, M. Martino30, N. Mary62, G. Matijevič89, T. Mazeh91, P.J. McMillan13, S.Messina52, A. Mestre113, D. Michalik13, N.R. Millar7, B.M.H. Miranda54, D. Molina10, R. Molinaro84, M.

Molinaro114, L. Molnár110, M. Moniez115, P. Montegriffo31, D. Monteiro21, R. Mor10, A. Mora47, R. Morbidelli27,T. Morel43, S. Morgenthaler116, T. Morley86, D. Morris41, A.F. Mulone30, T. Muraveva31, I. Musella84, J.

Narbonne62, G. Nelemans25, 24, L. Nicastro117, L. Noval62, C. Ordénovic14, J. Ordieres-Meré118, P. Osborne7, C.Pagani72, I. Pagano52, F. Pailler16, H. Palacin62, L. Palaversa8, P. Parsons22, T. Paulsen21, M. Pecoraro63, R.

Pedrosa119, H. Pentikäinen55, J. Pereira21, B. Pichon14, A.M. Piersimoni83, F.-X. Pineau45, E. Plachy110, G. Plum4,E. Poujoulet120, A. Prša121, L. Pulone79, S. Ragaini31, S. Rago27, N. Rambaux38, M. Ramos-Lerate122, P.

Ranalli13, G. Rauw43, A. Read72, S. Regibo24, F. Renk15, C. Reylé59, R.A. Ribeiro92, L. Rimoldini33, V. Ripepi84,A. Riva27, G. Rixon7, M. Roelens8, M. Romero-Gómez10, N. Rowell41, F. Royer4, A. Rudolph15, L. Ruiz-Dern4,G. Sadowski18, T. Sagristà Sellés6, J. Sahlmann12, J. Salgado57, E. Salguero57, M. Sarasso27, H. Savietto123, A.

Schnorhk21, M. Schultheis14, E. Sciacca52, M. Segol124, J.C. Segovia32, D. Segransan8, E. Serpell86, I-C. Shih4, R.Smareglia114, R.L. Smart27, C. Smith125, E. Solano71, 126, F. Solitro30, R. Sordo3, S. Soria Nieto10, J. Souchay36, A.Spagna27, F. Spoto14, U. Stampa6, I.A. Steele108, H. Steidelmüller11, C.A. Stephenson22, H. Stoev127, F.F. Suess7,M. Süveges33, J. Surdej43, L. Szabados110, E. Szegedi-Elek110, D. Tapiador128, 129, F. Taris36, G. Tauran62, M.B.

Taylor130, R. Teixeira85, D. Terrett34, B. Tingley131, S.C. Trager76, C. Turon4, A. Ulla132, E. Utrilla47, G.Valentini83, A. van Elteren2, E. Van Hemelryck39, M. van Leeuwen7, M. Varadi8, 110, A. Vecchiato27, J.

Veljanoski76, T. Via77, D. Vicente96, S. Vogt133, H. Voss10, V. Votruba95, S. Voutsinas41, G. Walmsley16, M.Weiler10, K. Weingrill89, D. Werner15, T. Wevers25, G. Whitehead15, Ł. Wyrzykowski7, 134, A. Yoldas7, M.Žerjal100, S. Zucker87, C. Zurbach48, T. Zwitter100, A. Alecu7, M. Allen1, C. Allende Prieto26, 135, 136, A.

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(Affiliations can be found after the references)

Received 2016-07-08; accepted 2016-08-18

ABSTRACT

Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006,following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payloadwere built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia DataProcessing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point,the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-skyscanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operatedto achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance ofthe mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. Wesummarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the datacan be retrieved from the Gaia Archive, which is available through the Gaia home page at http://www.cosmos.esa.int/gaia.

Key words. astrometry – parallaxes – proper motions – photometry – variable stars


Gaia Collaboration et al.: The Gaia mission

1. Introduction

Astrometry is the astronomical discipline concerned with the ac-curate measurement and study of the (changing) positions of ce-lestial objects. Astrometry has a long history (Perryman 2012)even before the invention of the telescope. Since then, advancesin the instrumentation have steadily improved the achievable an-gular accuracy, leading to a number of important discoveries:stellar proper motion (Halley 1717), stellar aberration (Bradley1727), nutation (Bradley 1748), and trigonometric stellar paral-lax (Bessel 1838; Henderson 1840; von Struve 1840). Obtain-ing accurate parallax measurements from the ground, however,remained extremely challenging owing to the difficulty to con-trol systematic errors and overcome the disturbing effects of theEarth’s atmosphere, and the need to correct the measured rela-tive to absolute parallaxes. Until the mid-1990s, for instance, thenumber of stars for which ground-based parallaxes were avail-able was limited to just over 8000 (van Altena et al. 1995, butsee Finch & Zacharias 2016).

This situation changed dramatically in 1997 with the Hippar-cos satellite of the European Space Agency (ESA), which mea-sured the absolute parallax with milli-arcsecond accuracy of asmany as 117 955 objects (ESA 1997). The Hipparcos data haveinfluenced many areas of astronomy (see the review by Perry-man 2009), in particular the structure and evolution of stars andthe kinematics of stars and stellar groups. Even with its limitedsample size and observed volume, Hipparcos also made signif-icant advances in our knowledge of the structure and dynamicsof our Galaxy, the Milky Way.

The ESA astrometric successor mission, Gaia, is expectedto completely transform the field. The main aim of Gaia is tomeasure the three-dimensional spatial and the three-dimensionalvelocity distribution of stars and to determine their astrophysicalproperties, such as surface gravity and effective temperature, tomap and understand the formation, structure, and past and futureevolution of our Galaxy (see the review by Bland-Hawthorn &Gerhard 2016). The Milky Way contains a complex mix of stars(and planets), interstellar gas and dust, and dark matter. Thesecomponents are widely distributed in age, reflecting their forma-tion history, and in space, reflecting their birth places and sub-sequent motions. Objects in the Milky Way move in a varietyof orbits that are determined by the gravitational force gener-ated by the integrated mass of baryons and dark matter, and havecomplex distributions of chemical-element abundances, reflect-ing star formation and gas-accretion history. Understanding allthese aspects in one coherent picture is the main aim of Gaia.Such an understanding is clearly also relevant for studies of thehigh-redshift Universe because a well-studied template galaxyunderpins the analysis of unresolved galaxies.

Gaia needs to sample a large, representative, part of theGalaxy, down to a magnitude limit of at least 20 in the GaiaG band to meet its primary science goals and to reach vari-ous (kinematic) tracers in the thin and thick disks, bulge, andhalo (Perryman et al. 2001, Table 1). For the 1000 millionstars expected down to this limit, Gaia needs to determine theirpresent-day, three-dimensional spatial structure and their three-dimensional space motions to determine their orbits and theunderlying Galactic gravitational potential and mass distribu-tion. The astrometry of Gaia delivers absolute parallaxes andtransverse kinematics (see Bailer-Jones 2015 on how to derivedistances from parallaxes). Complementary radial-velocity andphotometric information complete the kinematic and astrophys-ical information for a subset of the target objects, including in-terstellar extinctions and stellar chemical abundances.

Following the Rømer mission proposal from the early 1990s(see Høg 2008), the Gaia mission was proposed by Lennart Lin-degren and Michael Perryman in 1993 (for historical details, seeHøg 2014), after which a concept and technology study was con-ducted. The resulting science case and mission and spacecraftconcept are described in Perryman et al. (2001). In the earlyphases, Gaia was spelled as GAIA, for Global Astrometric Inter-ferometer for Astrophysics, but the spelling was later changedbecause the final design was non-interferometric and based onmonolithic mirrors and direct imaging and the final operatingprinciple was actually closer to a large Rømer mission than theoriginal GAIA proposal. After the selection of Gaia in 2000 asan ESA-only mission, followed by further preparatory studies,the implementation phase started in 2006 with the selection ofthe prime contractor, EADS Astrium (later renamed Airbus De-fence and Space), which was responsible for the developmentand implementation of the spacecraft and payload. Meanwhile,the complex processing and analysis of the mission data was en-trusted to the Data Processing and Analysis Consortium (DPAC),a pan-European, nationally funded collaboration of several hun-dred astronomers and software specialists. Gaia was launchedin December 2013 and the five-year nominal science operationsphase started in the summer of 2014, after a half-year period ofcommissioning and performance verification.

Unlike the Hipparcos mission, the Gaia collaboration doesnot have data rights. After processing, calibration, and valida-tion inside DPAC, data are made available to the world withoutlimitations; this also applies to the photometric and solar sys-tem object science alerts (Sect. 6.2). Several intermediate re-leases, with roughly a yearly cadence, have been defined andthis paper accompanies the first of these, referred to as GaiaData Release 1 (Gaia DR1; Gaia Collaboration et al. 2016). Thedata, accompanied by several query, visualisation, exploration,and collaboration tools, are available from the Gaia Archive(Salgado et al. 2016), which is reachable from the Gaia homepage at http://www.cosmos.esa.int/gaia and directly athttp://archives.esac.esa.int/gaia.

This paper is organised as follows: Section 2 summarises thescience goals of the mission. The spacecraft and payload designsand characteristics are described in Sect. 3. The launch and com-missioning phase are detailed in Sect. 4. Section 5 describes themission and mission operations. The science operations are sum-marised in Sect. 6. Section 7 outlines the structure and flow ofdata in DPAC. The science performance of the mission is dis-cussed in Sect. 8. A summary can be found in Sect. 9. All sec-tions are largely stand-alone descriptions of certain mission as-pects and can be read individually. The use of acronyms in thispaper has been minimised; a list can be found in Annex A.

2. Scientific goals

The science case for the Gaia mission was compiled in the year2000 (Perryman et al. 2001). The scientific goals of the designreference mission were relying heavily on astrometry, combinedwith its photometric and spectroscopic surveys. The astrometricpart of the science case remains unique, and so do the photomet-ric and spectroscopic data, despite various, large ground-basedsurveys having materialised in the last decade(s). The space en-vironment and design of Gaia enable a combination of accuracy,sensitivity, dynamic range, and sky coverage, which is practi-cally impossible to obtain with ground-based facilities target-ing photometric or spectroscopic surveys of a similar scientificscope. The spectra collected by the radial-velocity spectrome-ter (Sect. 3.3.7) have sufficient signal to noise for bright stars to



make the Gaia spectroscopic survey the biggest of its kind. Theastrometric part of Gaia is unique simply because global, micro-arcsecond astrometry is possible only from space. Therefore, thescience case outlined more than a decade ago remains largelyvalid and the Gaia data releases are still needed to address thescientific questions (for a recent overview of the expected yieldfrom Gaia, see Walton et al. 2014). A non-exhaustive list of sci-entific topics is provided in this section with an outline of themost important Gaia contributions.

2.1. Structure, dynamics, and evolution of the Galaxy

The fundamental scientific-performance requirements for Gaiastem, to a large extent, from the main scientific target of the mis-sion: the Milky Way galaxy. Gaia is built to address the questionof the formation and evolution of the Galaxy through the analy-sis of the distribution and kinematics of the luminous and darkmass in the Galaxy. By also providing measurements to deducethe physical properties of the constituent stars, it is possible tostudy the structure and dynamics of the Galaxy. Although theGaia sample will only cover about 1% of the stars in the MilkyWay, it will consist of more than 1000 million stars covering alarge volume (out to many kpc, depending on spectral type), al-lowing thorough statistical analysis work to be conducted. Thedynamical range of the Gaia measurements facilitates reachingstars and clusters in the Galactic disk out to the Galactic centreas well as far out in the halo, while providing extremely high ac-curacies in the solar neighbourhood. In addition to using stars asprobes of Galactic structure and the local, Galactic potential inwhich they move, stars can also be used to map the interstellarmatter. By combining extinction deduced from stars, it is pos-sible to construct the three-dimensional distribution of dust inour Galaxy. In this way, Gaia will address not only the stellarcontents, but also the interstellar matter in the Milky Way.

2.2. Star formation history of the Galaxy

The current understanding of galaxy formation is based on acombination of theories and observations, both of (high-redshift)extragalactic objects and of individual stars in our Milky Way.The Milky Way galaxy provides the single possibility to studydetails of the processes, but the observational challenges are dif-ferent in comparison with measuring other galaxies. From ourperspective, the Galaxy covers the full sky, with some compo-nents far away in the halo requiring sensitivity, while stars in thecrowded Galactic centre region require spatial resolving power.Both these topics can be addressed with the Gaia data. Gaia dis-tances will allow the derivation of absolute luminosities for starswhich, combined with metallicities, allow the derivation of ac-curate individual ages, in particular for old subgiants, which areevolving from the main-sequence turn-off to the bottom of thered giant branch. By combining the structure and dynamics ofthe Galaxy with the information of the physical properties of theindividual stars and, in particular, ages, it is possible to deducethe star formation histories of the stellar populations in the MilkyWay.

2.3. Stellar physics and evolution

Distances are one of the most fundamental quantities needed tounderstand and interpret various astronomical observations ofstars. Yet direct distance measurement using trigonometric par-allax of any object outside the immediate solar neighbourhood

or not emitting in radio wavelengths is challenging from theground. The Gaia revolution will be in the parallaxes, with hun-dreds of millions being accurate enough to derive high-qualitycolour-magnitude diagrams and to make significant progress instellar astrophysics. The strength of Gaia is also in the number ofobjects that are surveyed as many phases of stellar evolution arefast. With 1000 million parallaxes, Gaia will cover most phasesof evolution across the stellar-mass range, including pre-main-sequence stars and (chemically) peculiar objects. In addition toparallaxes, the homogeneous, high-accuracy photometry will al-low fine tuning of stellar models to match not only individualobjects, but also star clusters and populations as a whole. Thecombination of Gaia astrometry and photometry will also con-tribute significantly to star formation studies.

2.4. Stellar variability and distance scale

On average, each star is measured astrometrically ∼70 times dur-ing the five-year nominal operations phase (Sect. 5.2). At eachepoch, photometric measurements are also made: ten in the GaiaG broadband filter and one each with the red and blue photome-ter (Sect. 8.2). For the variable sky, this provides a systematicsurvey with the sampling and cadence of the scanning law ofGaia (Sect. 5.2). This full-sky survey will provide a census ofvariable stars with tens of millions of new variables, includingrare objects. Sudden photometric changes in transient objectscan be captured and the community can be alerted for follow-upobservations. Pulsating stars, especially RR Lyrae and Cepheids,can easily be discovered from the Gaia data stream allowing,in combination with the parallaxes, calibration of the period-luminosity relations to better accuracies, thereby improving thequality of the cosmic-distance ladder and scale.

2.5. Binaries and multiple stars

Gaia is a powerful mission to improve our understanding ofmultiple stars. The instantaneous spatial resolution, in the scan-ning direction, is comparable to that of the Hubble Space Tele-scope and Gaia is surveying the whole sky. In addition to re-solving many binaries, all instruments in Gaia can complementour understanding of multiple systems. The astrometric wob-bles of unresolved binaries, seen superimposed on parallacticand proper motions, can be used to identify multiple systems.Periodic changes in photometry can be used to find (eclipsing)binaries and an improved census of double-lined systems basedon spectroscopy will follow from the Gaia data. It is again thelarge number of objects that Gaia will provide that will help ad-dress the fundamental questions of mass distributions and orbitaleccentricities among binaries.

2.6. Exoplanets

From the whole spectrum of scientific topics that Gaia can ad-dress, the exoplanet research area has been the most dynamicin the past two decades. The field has expanded from hot, gi-ant planets to smaller planets, to planets further away from theirhost star, and to multiple planetary systems. These advance-ments have been achieved both with space- and ground-basedfacilities. Nevertheless, the Gaia astrometric capabilities remainunique, probing a poorly explored area in the parameter spaceof exoplanetary systems and providing astrophysical parame-ters not obtainable by other means. A strong point of Gaia inthe exoplanet research field is the provision of an unbiased,



volume-limited sample of Jupiter-mass planets in multiyear or-bits around their host stars. These are logical prime targets forfuture searches of terrestrial-mass exoplanets in the habitablezone in an orbit protected by a giant planet further out. In ad-dition, the astrometric data of Gaia allow actual masses (ratherthan lower limits) to be measured. Finally, the data of Gaia willprovide the detailed distributions of giant exoplanet properties(including the giant planet - brown dwarf transition regime) asa function of stellar-host properties with unprecedented resolu-tion.

2.7. Solar system

Although Gaia is designed to detect and observe stars, it willprovide a full census of all sources that appear point-like on thesky. The movement of solar system objects with respect to thestars smears their images and makes them less point-like. Aslong as this smearing is modest, Gaia will still detect the object.The most relevant solar system object group for Gaia are aster-oids. Unlike planets, which are too big in size (and, in addition,sometimes too bright) to be detected by Gaia, asteroids remaintypically point-like and have brightness in the dynamical rangeof Gaia. Gaia astrometry and photometry will provide a censusof orbital parameters and taxonomy in a single, homogeneousphotometric system. The full-sky coverage of Gaia will also pro-vide this census far away from the ecliptic plane as well as forlocations inside the orbit of the Earth. An alert can be made ofnewly discovered asteroids to trigger ground-based observationsto avoid losing the object again. For near-Earth asteroids, Gaiais not going to be very complete as the high apparent motion ofsuch objects often prevents Gaia detection, but in those caseswhere Gaia observations are made, the orbit determination canbe very precise. Gaia will provide fundamental mass measure-ments of those asteroids that experience encounters with othersolar system bodies during the Gaia operational lifetime.

2.8. The Local Group

In the Local Group, the spatial resolution of Gaia is sufficientto resolve and observe the brightest individual stars. Tens of Lo-cal Group galaxies will be covered, including the Andromedagalaxy and the Magellanic Clouds. While for the faintest dwarfgalaxies only a few dozen of the brightest stars are observed,this number increases to thousands and millions of stars in An-dromeda and the Large Magellanic Cloud, respectively. In dwarfspheroidals such as Fornax, Sculptor, Carina, and Sextans, thou-sands of stars will be covered. A major scientific goal of Gaiain the Local Group concerns the mutual, dynamical interactionof the Magellanic Clouds and the interaction between the Cloudsand the Galaxy. In addition to providing absolute proper motionsfor transverse-velocity determination, needed for orbits, it is pos-sible to explore internal stellar motions within dwarf galaxies.These kinds of data may reveal the impact of dark matter, amongother physical processes in the host galaxy, to the motions of itsstars.

2.9. Unresolved galaxies, quasars, and the reference frame

Gaia will provide a homogeneous, magnitude-limited sample ofunresolved galaxies. For resolved galaxies, the sampling func-tion is complicated as the onboard detection depends on the con-trast between any point-like, central element (bulge) and any ex-tended structure, convolved with the scanning direction. For un-

resolved galaxies, the most valuable measurements are the pho-tometric observations. Millions of galaxies across the whole skywill be measured systematically. As the same Gaia system isused for stellar work, one can anticipate that, in the longer term,the astrophysical interpretation of the photometry of extragalac-tic objects will be based on statistically sound fundaments ob-tained from Galactic studies. Quasars form a special category ofextragalactic sources for Gaia as not only their intrinsic prop-erties can be studied, but they can also be used in comparisonsof optical and radio reference frames. Such a comparison will,among others, answer questions of the coincidence of quasar po-sitions across different wavelengths.

2.10. Fundamental physics

As explained in Sect. 7.3, relativistic corrections are part of theroutine data processing for Gaia. Given the huge number of mea-surements, it is possible to exploit the redundancy in these cor-rections to conduct relativity tests or to use (residuals of) theGaia data in more general fundamental-physics experiments.Specifically for light bending, it is possible to determine the γparameter in the parametrised post-Newtonian formulation veryprecisely. Another possible experiment is to explore light bend-ing of star images close to the limb of Jupiter to measure thequadrupole moment of the gravitational field of the giant planet.A common element in all fundamental physics tests using Gaiadata is the combination of large sets of measurements. This ismeaningful only when all systematic effects are under control,down to micro-arcsecond levels. Therefore, Gaia results for rela-tivistic tests can be expected only towards the end of the mission,when all calibration aspects have been handled successfully.

3. Spacecraft and payload

The Gaia satellite (Fig. 1) has been built under an ESA con-tract by Airbus Defence and Space (DS, formerly known as As-trium) in Toulouse (France). It consists of a payload module(PLM; Sect. 3.3), which was built under the responsibility ofAirbus DS in Toulouse; a mechanical service module (M-SVM;Sect. 3.2), which was built under the responsibility of Airbus DSin Friedrichshafen (Germany); and an electrical service module(E-SVM; Sect. 3.2), which was built under the responsibility ofAirbus DS in Stevenage (United Kingdom).

3.1. Astrometric measurement principle and overall designconsiderations

The measurement principle of Gaia is derived from the global-astrometry concept successfully demonstrated by the ESA astro-metric predecessor mission, Hipparcos (Perryman et al. 1989).This principle of scanning space astrometry (Lindegren & Bas-tian 2011) relies on a slowly spinning satellite that measures thecrossing times of targets transiting the focal plane. These ob-servation times represent the one-dimensional, along-scan (AL)stellar positions relative to the instrument axes. The astrometriccatalogue is built up from a large number of such observationtimes, by an astrometric global iterative solution (AGIS) process(e.g. Lindegren et al. 2012, 2016), which also involves a simul-taneous reconstruction of the instrument pointing (attitude) as afunction of time, and of the optical mapping of the focal planedetector elements (pixels) through the telescope(s) onto the ce-lestial sphere (geometric calibration). The fact that the nuisanceparameters to describe the attitude and geometric calibration are



Fig. 2. Measurable, along-scan (AL) angle between the stars at P and F depends on their parallaxes $P and $F in different ways, depending on theposition of the Sun. This allows us to determine their absolute parallaxes, rather than just the relative parallax $P −$F. Wide-angle measurementsalso guarantee a distortion-free and rigid system of coordinates and proper motions over the whole sky. Image from Lindegren & Bastian (2011).

derived simultaneously with the astrometric source parametersfrom the regular observation data alone (without special, cali-bration data) means that Gaia is a self-calibrating mission.

Following in the footsteps of Hipparcos, Gaia is equippedwith two fields of view, separated by a constant, large angle (thebasic angle) on the sky along the scanning circle. The two view-ing directions map the images onto a common focal plane suchthat the observation times can be converted into small-scale an-gular separations between stars inside each field of view andlarge-scale separations between objects in the two fields of view.Because the parallactic displacement (parallax factor) of a givensource is proportional to sin θ, where θ is the angle between thestar and the Sun, the parallax factors of stars inside a given fieldof view are nearly identical, suggesting only relative parallaxescould be measured. However, although scanning space astrome-try makes purely differential measurements, absolute parallaxescan be obtained because the relative parallactic displacementscan be measured between stars that are separated on the sky bya large angle (the basic angle) and, hence, have a substantiallydifferent parallax factor. To illustrate this further, consider an ob-server at one astronomical unit from the Sun. The apparent shiftof a star owing to its parallax $ then equals $ sin θ and is di-rected along the great circle from the star towards the Sun. Asshown in Fig. 2 (left panel), the measurable, along-scan parallaxshift of a star at position F (for following field of view) equals$F sin θ sinψ = $F sin ξ sin Γ, where ξ is the angle between theSun and the spin axis (the solar-aspect angle). At the same time,the measurable, along-scan parallax shift of a star at position P(for preceding field of view) equals zero. The along-scan mea-surement of F relative to P therefore depends on $F but not on$P, while the reverse is true at a different time (right panel). So,scanning space astrometry delivers absolute parallaxes.

The sensitivity of Gaia to parallax, which means the measur-able, along-scan effect, is proportional to sin ξ sin Γ. This has thefollowing implications:

– Ideally, Γ equals 90◦. However, when scanning more or lessalong a great circle (as during a day or so), the accuracy withwhich the one-dimensional positions of stars along the greatcircle can be derived, as carried out in the one-day itera-tive solution (ODAS) as part of continuous payload healthmonitoring (Sect. 6.3), is poor when Γ = 360◦ × m/n forsmall integer values of m and n (Lindegren & Bastian 2011);this can be understood in terms of the connectivity of stars

along the circle (Makarov 1998). Taking this into account,several acceptable ranges for the basic angle remain, for in-stance 99◦.4±0◦.1 and 106◦.5±0◦.1. Telescope accommodationaspects identified during industrial studies favoured 106◦.5as the design value adopted for Gaia. During commission-ing, using Tycho-2 stars, the actual in-flight value was mea-sured to be 1′′.3 larger than the design value. For the global-astrometry concept to work, it is important to either have anextremely stable basic angle (i.e. thermally stable payload)on timescales of a few revolutions and/or to continuouslymeasure its variations with high precision. Therefore, Gaiais equipped with a basic angle monitor (Sect. 3.3.4).

– Ideally, ξ equals 90◦. However, this would mean that sun-light would enter the telescope apertures. To ensure optimumthermal stability of the payload, in view of minimising ba-sic angle variations, it is clear that ξ should be chosen to beconstant. For Gaia, ξ = 45◦ represents the optimal point be-tween astrometric-performance requirements, which call fora large angle, and implementation constraints, such as therequired size of the sunshield to keep the payload in perma-nent shadow and solar-array-efficiency and sizing arguments,which call for a small angle.

Finally, the selected spin rate of Gaia, nominally 60′′ s−1 (actual,in-flight value: 59′′.9605 s−1), is a complex compromise involv-ing arguments on mission duration and these arguments: revisitfrequency, attitude-induced point spread function blurring dur-ing detector integration, signal-to-noise ratio considerations, fo-cal plane layout and detector characteristics, and telemetry rate.

3.2. Service module

The mechanical service module comprises all mechanical, struc-tural, and thermal elements supporting the instrument and thespacecraft electronics. The service module physically accom-modates several electronic boxes including the video process-ing units (Sect. 3.3.8), payload data-handling unit (Sect. 3.3.9),and clock distribution unit (Sect. 3.3.10), which functionally be-long to the payload module but are housed elsewhere in viewof the maintenance of the thermal stability of the payload. Theservice module also includes the chemical and micro-propulsionsystems, deployable-sunshield assembly, payload thermal tent,solar-array panels, and electrical harness. The electrical servicesalso support functions to the payload and spacecraft for attitude



Fig. 1. Exploded, schematic view of Gaia. (a) Payload thermal tent(Sect. 3.3); (b) payload module: optical bench, telescopes, instruments,and focal plane assembly (Sect. 3.3); (c) service module (structure):also housing some electronic payload equipment, e.g. clock distri-bution unit, video processing units, and payload data-handling unit(Sect. 3.2); (d) propellant systems (Sect. 3.2.1); (e) phased-array an-tenna (Sect. 3.2.2); and (f) deployable sunshield assembly, includingsolar arrays (Sect. 3.2). Credit: ESA, ATG Medialab.

control, electrical power control and distribution, central datamanagement, and communications with the Earth through lowgain antennae and a high-gain phased-array antenna for sciencedata transmission. In view of their relevance to the science per-formance of Gaia, the attitude and orbit control and phased-arrayantenna subsystems are described in more detail below.

3.2.1. Attitude and orbit control

The extreme centroiding needs of the payload make stringentdemands on satellite attitude control over the integration time ofthe payload detectors (of order a few seconds). This requires inparticular that rate errors and relative-pointing errors be kept atthe milli-arcsecond per second and milli-arcsecond level, respec-tively. These requirements prohibit the use of moving parts, suchas conventional reaction wheels, on the spacecraft, apart frommoving parts within thrusters. The attitude- and orbit-control

subsystem (AOCS) is therefore based on a custom design (e.g.Chapman et al. 2011; Risquez et al. 2012) including varioussensors and actuators. The sensors include two autonomous startrackers (used in cold redundancy), three fine Sun sensors usedin hot redundancy (i.e. with triple majority voting), three fibre-optic gyroscopes (internally redundant), and low-noise rate dataprovided by the payload through measurements of star transitspeeds through the focal plane. Gaia contains two flavours of ac-tuators: two sets of eight bi-propellant (NTO oxidiser and MMHfuel) newton-level thrusters (used in cold redundancy) formingthe chemical-propulsion subsystem (CPS) for spacecraft ma-noeuvres and back-up modes, including periodic orbit mainte-nance (Sect. 5.3.2); and two sets of six proportional-cold-gas,micro-newton-level thrusters forming the micro-propulsion sub-system (MPS) for fine attitude control required for nominal sci-ence operations. In nominal operations (AOCS normal mode),only the star-tracker and payload-rate data are used in a closed-loop, three-axes control with the MPS thrusters, which are oper-ated with a commanded thrust bias; the other sensors are onlyused for failure detection, isolation, and recovery. Automatic,bi-directional mode transitions between several coarse and finepointing modes have been implemented to allow efficient oper-ation and autonomous settling during transient events, such asmicro-meteoroid impacts (Sect. 5.1).

3.2.2. Phased-array antenna

Extreme centroiding requirements of the payload prohibit theuse of a conventional, mechanically steered dish antenna for sci-ence data downlink because moving parts in Gaia would causeunacceptable degradation of the image quality through micro-vibrations. Gaia therefore uses a high-gain phased-array antenna(PAA), allowing the signal to be directed towards Earth as thespacecraft rotates (and as it moves through its orbit around theL2 Lagrange point; Sect. 5.1) by means of electronic beam steer-ing (phase shifting). The antenna is mounted on the Sun- andEarth-pointing face of the service module, which is perpendic-ular to the rotation axis. The radiating surface resembles a 14-sided, truncated pyramid. Each of the 14 facets has two subar-rays and each comprises six radiating elements. Each subarraysplits the incoming signal to provide the amplitude weightingthat determines the radiation pattern of the subarray. The overallantenna radiation pattern is obtained by combining the radiationpatterns from the 14 subarrays. The equivalent isotropic radiatedpower (EIRP) of the antenna exceeds 32 dBW over most of the30◦ elevation range (Sect. 5.1), allowing a downlink informationdata rate of 8.7 megabits per second (Sect. 5.3.1) in the X band.The phased-array antenna is also used with orbit reconstructionmeasurements made from ground (Sect. 5.3.2).

3.3. Payload module

The payload module (Fig. 3) is built around an optical bench thatprovides structural support for the two telescopes (Sect. 3.3.1)and the single integrated focal plane assembly (Sect. 3.3.2) thatcomprises, besides wave-front-sensing and basic angle metrol-ogy (Sects. 3.3.3 and 3.3.4), three science functions: astrom-etry (Sect. 3.3.5), photometry (Sect. 3.3.6), and spectroscopy(Sect. 3.3.7). The payload module is mounted on top of the ser-vice module via two (parallel) sets of three, V-shaped bipods.The first set of launch bipods is designed to withstand me-chanical launch loads and these have been released in orbitto a parking position to free the second set of glass-fibre-



Fig. 3. Schematic payload overview without protective tent. Most electronic boxes, e.g. clock distribution unit, video processing units, or payloaddata-handling unit, are physically located in the service module and hence not visible here. Credit: ESA.

reinforced polymer in-orbit bipods; the latter have low con-ductance and thermally decouple the payload from the servicemodule. The payload is covered by a thermal tent based on acarbon-fibre-reinforced-polymer and aluminium sandwich struc-ture with openings for the two telescope apertures and for thefocal plane, warm-electronics radiator. The tent provides ther-mal insulation from the external environment and protects thefocal plane and mirrors from micro-meteoroid impacts. The pay-load module furthermore contains the spacecraft master clock(Sect. 3.3.10) and all necessary electronics for managing the in-strument operation and processing and storing the science data(Sects. 3.3.8 and 3.3.9); these units, however, are physically lo-cated in the service module.

3.3.1. Telescope

Gaia is equipped with two identical, three-mirror anastigmatic(TMA) telescopes, with apertures of 1.45 m × 0.50 m pointingin directions separated by the basic angle (Γ = 106◦.5). Thesetelescopes and their associated viewing directions (lines of sight)are often referred to as 1 and 2 or preceding and following, re-spectively, where the latter description refers to objects that arescanned first by the preceding and then by the following tele-scope. In order to allow both telescopes to illuminate a sharedfocal plane, the beams are merged into a common path at theexit pupil and then folded twice to accommodate the 35 m focallength. The total optical path hence encounters six reflectors: thefirst three (M1–M3 and M1’–M3’) form the TMAs, the fourthis a flat beam combiner (M4 and M4’), and the final two are flatfolding mirrors for the common path (M5–M6). All mirrors havea protected silver coating ensuring high reflectivity and a broadbandpass, starting around 330 nm. Asymmetric optical aberra-tions in the optics cause tiny yet significant chromatic shifts ofthe diffraction images and thus of the measured star positions.These systematic displacements are calibrated out as part of theon-ground data processing (Lindegren et al. 2016) using colour

information provided by the photometry collected for each ob-ject (Sect. 3.3.6).

The telescopes are mounted on a quasi-octagonal opticalbench of ∼3 m in diameter. The optical bench (composed of 17segments, brazed together) and all telescope mirrors are madeof sintered silicon carbide. This material combines high specificstrength and thermal conductivity, providing optimum passivethermo-elastic stability (but see Sect. 4.2).

The (required) optical quality of Gaia is high, with a totalwave-front error budget of 50 nm. To reach this number in orbit,after having experienced launch vibrations and gravity release,alignment and focussing mechanisms have been incorporated atthe secondary (M2) mirrors. These devices, called M2 mirrormechanisms (M2MMs), contain a set of actuators that are ca-pable of orienting the M2 mirrors with five degrees of freedom,which is sufficient for a rotationally symmetric surface. The in-orbit telescope focussing is detailed in Mora et al. (2014b, seealso Sect. 6.4) and has been inferred from a combination ofthe science data themselves (size and shape of the point spreadfunction) combined with data from the two wave-front sensors(WFSs; Sect. 3.3.3).

3.3.2. Focal plane assembly

The focal plane assembly of Gaia (for a detailed description,see Kohley et al. 2012; Crowley et al. 2016b) is commonto both telescopes and has five main functions: (i) metrology(wave-front sensing [WFS] and basic angle monitoring [BAM];Sects. 3.3.3 and 3.3.4), (ii) object detection in the sky map-per (SM; Sect. 3.3.5), (iii) astrometry in the astrometric field(AF; Sect. 3.3.5), (iv) low-resolution spectro-photometry usingthe blue and red photometers (BP and RP; Sect. 3.3.6), and(v) spectroscopy using the radial-velocity spectrometer (RVS;Sect. 3.3.7). The focal plane is depicted in Figure 4 and car-ries 106 charge-coupled device (CCD) detectors, arranged in amosaic of 7 across-scan rows and 17 along-scan strips, with a



total of 938 million pixels. These detectors come in three differ-ent types, which are all derived from CCD91-72 from e2v tech-nologies Ltd: the default, broadband CCD; the blue(-enhanced)CCD; and the red(-enhanced) CCD. Each of these types has thesame architecture but differ in their anti-reflection coating andapplied surface-passivation process, their thickness, and the re-sistivity of their silicon wafer. The broadband and blue CCDs areboth 16 µm thick and are manufactured from standard-resistivitysilicon (100 Ω cm); they differ only in their anti-reflection coat-ing, which is optimised for short wavelengths for the blue CCD(centred on 360 nm) and optimised to cover a broad bandpass forthe broadband CCD (centred on 650 nm). The red CCD, in con-trast, is based on high-resistivity silicon (1000 Ω cm), is 40 µmthick, and has an anti-reflection coating optimised for long wave-lengths (centred on 750 nm). The broadband CCD is used in SM,AF, and the WFS. The blue CCD is used in BP. The red CCD isused in BAM, RP, and the RVS.

The detectors (Fig. 5; Crowley et al. 2016b) are back-illuminated, full-frame devices with an image area of 4500 linesalong-scan and 1966 columns across-scan; each pixel is 10 µm× 30 µm in size (corresponding to 58.9 mas × 176.8 mas on thesky), balancing along-scan resolution and pixel full-well capac-ity (around 190 000 e−). All CCDs are operated in time-delayedintegration (TDI) mode to allow collecting charges as the ob-ject images move over the CCD and transit the focal plane asa result of the spacecraft spin. The fundamental line shift pe-riod of 982.8 µs is derived from the spacecraft atomic masterclock (Sect. 3.3.10); the focus of the telescopes is adjusted toensure that the speed of the optical images over the CCD sur-face matches the fixed speed at which the charges are clockedinside the CCD. The 10 µm pixel in the along-scan direction isdivided into four clock phases to minimise the blurring effect ofthe discrete clocking operation on the along-scan image qual-ity. The integration time per CCD is 4.42 s, corresponding tothe 4500 TDI lines along-scan; actually, only 4494 of these linesare light sensitive. The CCD image area is extended along-scanby a light-shielded summing well with adjacent transfer gate tothe two-phase serial (readout) register, permitting TDI clock-ing (and along-scan binning) in parallel with register readout.The serial register ends with a non-illuminated post-scan pixeland begins with several non-illuminated pre-scan pixels that areconnected to a single, low-noise output-amplifier structure, en-abling across-scan binning on the high-charge-handling capacity(∼240 000 e−) output node. Total noise levels of the full detectionchain vary from 3 to 5 electrons RMS per read sample (exceptfor SM and AF1, which have values of 11 and 8 electrons RMS,respectively), depending on the CCD operating mode.

The CCDs are composed of 18 stitch blocks, originatingfrom the mask employed in the photo-lithographic produc-tion process with eight across-scan and one along-scan bound-aries (Fig. 5). Each block is composed of 250 columns (and2250 lines) except for the termination blocks, which have 108columns. Whereas pixels inside a given stitch block are typi-cally well-aligned, small misalignments between adjacent stitchblocks necessitate discontinuities in the small-scale geometriccalibration of the CCDs (Lindegren et al. 2016). The mask-positioning accuracy for the individual stitch blocks also pro-duces discontinuities in several response vectors, such as charge-injection non-uniformity and column-response non-uniformity.At distinct positions along the 4500 TDI lines, a set of 12 specialelectrodes (TDI gates) are connected to their own clock driver.In normal operation, these electrodes are clocked synchronouslywith the other electrodes. These TDI-gate electrodes can, how-ever, be temporarily (or permanently) held low such that charge

Fig. 4. Schematic image of the focal plane assembly, superimposed on areal picture of the CCD support structure (with a human hand to indicatethe scale), with Gaia-specific terminology indicated (e.g. CCD strip androw, TDI line and pixel column). The RVS spectrometer CCDs are dis-placed vertically (in the across-scan direction) to correct for a lateraloptical displacement of the light beam caused by the RVS optics suchthat the RVS CCD rows are aligned with the astrometric and photomet-ric CCD rows on the sky; the resulting semi-simultaneity of the astro-metric, photometric, and spectroscopic transit data is advantageous forstellar variability, science alerts, spectroscopic binaries, etc. Image fromde Bruijne et al. (2010a); Kohley et al. (2012), courtesy Airbus DS andBoostec Industries.

transfer over these lines in the image area is inhibited and TDIintegration time is effectively reduced to the remaining numberof lines between the gate and the readout register. While the full4500-lines integration is normally used for faint objects, TDIgates are activated for bright objects to limit image-area satu-ration. Available integration times are 4500, 2900, 2048, 1024,512, 256, 128, 64, 32, 16, 8, 4, and 2 TDI lines. The choiceof which gate to activate is user-defined, based on configurablelook-up tables depending on the brightness of the object, theCCD, the field of view, and the across-scan pixel coordinate. Be-cause the object brightness that is measured on board in the skymapper (Sect. 3.3.9) has an error of a few tenths of a magnitude,a given (photometrically-constant) star, in particular when closein brightness to a gate-transition magnitude, is not always ob-served with the same gate on each transit. This mixing of gatesis beneficial for the astrometric and photometric calibrations ofthe gated instruments.

The Gaia CCDs are n-channel devices, i.e. built on p-typesilicon wafers with n-type channel doping. Displacement dam-age in the silicon lattice, caused by non-ionising irradiation, cre-ates defect centres (traps) in the channel that act as electron trapsduring charge transfer, leading to charge-transfer inefficiency(CTI). Under the influence of radiation, n-channel devices aresusceptible to develop a variety of trap species with release-timeconstants varying from micro-seconds to tens of seconds. Traps,in combination with TDI operation, affect the detailed shape ofthe point spread function of all instruments in subtle yet sig-nificant ways through continuous trapping and de-trapping (Hollet al. 2012b; Prod’homme et al. 2012), removing charge from theleading edge and releasing it in the trailing edge of the imagesand spectra. The resulting systematic biases of the image cen-troids and the spectra will be calibrated in the on-ground dataprocessing, for instance using a forward-modelling approachbased on a charge-distortion model (CDM; Short et al. 2013).



Fig. 5. Schematic view of a Gaia CCD detector. Stars move from leftto right in the along-scan direction (yellow arrow). Charges in the read-out register are clocked from bottom to top. The first line of the CCD(left) contains the charge-injection structure (red). The last line of theCCD before the readout register (right) contains the summing well andtransfer gate (blue). Dashed, grey lines indicate stitch-block boundaries.Solid, green vertical lines indicate TDI gates (the three longest lines arelabelled at the top of the CCD). The inset shows some details of anindividual pixel. See Sect. 3.3.2 for details.

The CCDs are passively cooled to 163 K to reduce dark currentand minimise (radiation-induced) along- and across-scan CTI.To further mitigate CTI, two features have been implemented inthe detector design: first, a charge-injection structure to period-ically inject a line of electronic charge into the last CCD line(furthest from the readout register), which is then transferred bythe TDI clocks through the device image area along with starimages, thereby (temporarily) filling traps; and, second, a sup-plementary buried channel (SBC; Seabroke et al. 2013) in eachCCD column to reduce the effect of CTI for small charge pack-ets by confining the transfer channel in the across-scan direction,thereby exposing the signal to fewer trapping centres.

The CCDs are mounted on a support structure integrated intoa cold-radiator box, which provides a radiative surface to theinternal payload cavity (which is around 120 K), CCD shield-ing against radiation, and mounting support for the photome-ter prisms (Sect. 3.3.6) and straylight vanes and baffles. EachCCD has its own proximity-electronics module (PEM), locatedbehind the CCD (support structure) on the warm side of the fo-cal plane assembly. Power from the warm electronics is dissi-pated directly to cold space through an opening in the thermaltent that encloses the payload module. Low-conductance bipodsand thermal shields provide thermal isolation between the warmand cold parts of the focal plane assembly. The PEMs providedigital correlated double sampling and contain an input stage,a low-noise pre-amplifier with two programmable gain stages(low gain for full dynamic range or high gain for limited dy-namic range and minimum noise), a bandwidth selector, and a

16-bit analogue-to-digital converter (ADC). The PEMs allow foradjustment of the CCD operating points, which might becomenecessary at some point as a result of flat-band voltage shifts in-duced by ionising radiation (monitoring of which is described inSect. 6.4). All CCD-PEM couples of a given row of CCDs areconnected through a power- and command-distribution intercon-nection module to a video processing unit (VPU; Sect. 3.3.8),which is in charge of generating the CCD commanding and ac-quiring the science data.

Operating the 100+ CCDs, comprising nearly a billion pix-els, in TDI mode with a line period of ∼1 ms would generate adata rate that is orders of magnitude too high to be transmitted toground. Three onboard reduction processes are hence applied:

1. Not all pixel data are read from the CCDs but only smallareas, windows, around objects of interest; remaining pixeldata are flushed at high speed in the serial register. This hasan associated advantage of decreased read noise for the de-sired pixels;

2. The two-dimensional images (windows) are, except forbright stars, binned in the across-scan direction, neverthe-less preserving the scientific information content (timing /along-scan centroid, total intensity / magnitude, and spectralinformation);

3. The resulting along-scan intensity profiles, such as line-spread functions or spectra, are compressed on board withoutloss of information; the typical gain in data volume is a factor2.0–2.5.

Windows are assigned by the VPU on-the-fly following au-tonomous object detection in the sky mapper (Sect. 3.3.5) andtherefore the readout configuration of flushed and read (binnedor unbinned) pixels is constantly changing with the sky passingby. This, together with the high-frequency pixel shift in the read-out register and the interleaving of the TDI image-area clocking,causes a systematic fluctuation of the electronic bias level alongthe same TDI line during readout (known as the [CCD-]PEM[bias] non-uniformity), which is calibrated on ground (Fabriciuset al. 2016).

3.3.3. Wave-front sensor

The focal plane of Gaia is equipped with two wave-front sen-sors (WFSs; Vosteen et al. 2009). These allow monitoring of theoptical performance of the telescopes and deriving informationto drive the M2 mirror mechanisms to (re-)align and (re-)focusthe telescopes (Sect. 3.3.1). The WFSs are of Shack-Hartmanntype and sample the output pupil of each telescope with an ar-ray of 3 × 11 microlenses. These microlenses focus the light ofbright stars transiting the focal plane on a CCD. Comparisonof the stellar spot pattern with the pattern of a built-in calibra-tion source (used during initial tests after launch) and with thepattern of stars acquired after achieving best focus (used after-wards) allows reconstruction of the wave front in the form of aseries of two-dimensional Legendre polynomials (Zernike poly-nomials are less appropriate for a rectangular pupil; Mora & Vos-teen 2012). The location of the microlenses within the telescopepupils is inferred from the flux collected by the surrounding,partially-illuminated lenslets. The M2 mirror-mechanism actua-tions are derived using a telescope-alignment tool based on mod-elled sensitivities for each degree of freedom. The number ofactuators to use and the weight given to each Legendre coeffi-cient are adjustable. The corrections applied so far after each de-contamination campaign (Sect. 6.4) have consisted of pure focusdisplacements.



3.3.4. Basic angle monitor

As explained in Sect. 3.1, the measurement principle of Gaiarelies on transforming transit-time differences between stars ob-served in both telescopes into angular measurements. This re-quires the basic angle Γ between the two fields of view eitherto be stable or to be monitored continuously at µas level andobserved variations corrected as part of the data processing.Whereas low-frequency variations that are longer than, for in-stance two spin periods, i.e. 12 h (Sect. 5.2), are absorbed in thegeometric instrument calibration (Lindegren et al. 2016), short-term variations, on timescales of minutes to hours, are non-trivialto calibrate and can introduce systematic errors in the astromet-ric results. In particular, a Sun-synchronous, periodic basic anglevariation is known to be (nearly) fully degenerate with the par-allax zero point (e.g. Lindegren et al. 1992). For this reason, thepayload of Gaia was designed to be stable on these timescales towithin a few µas (but see Sect. 4.2) and arguably carries the mostprecise interferometric metrology system ever flown, the basicangle monitor (BAM; e.g. Meijer et al. 2009; Gielesen et al.2013; Mora et al. 2014b). The BAM is composed of two opti-cal benches fed by a common laser source that introduces twoparallel, collimated beams per telescope. The BAM creates oneYoung-type fringe pattern per telescope in the same detector inthe focal plane. The relative along-scan displacement betweenthe two fringe patterns allows monitoring of the changes in theline of sight of each telescope and, thus, the basic angle. The(short-term) precision achieved in the differential measurementis 0.5 µas each 10–15 min, which corresponds to picometer dis-placements of the primary mirrors. A spare laser unit is kept incold redundancy in case the primary source were to fail. TheBAM exposures are continuously acquired with a period of 23 s(18.7 s stare-mode integration plus 4.4 s TDI-mode readout). Aforward-modelling approach, which is based on a mathematicalmodel representing the BAM image that is fitted using a least-squares algorithm, is applied in the daily preprocessing pipeline(Fabricius et al. 2016) to monitor basic angle variations; basicangle variations are also monitored independently on a daily ba-sis using cross-correlation techniques.

3.3.5. Astrometric instrument

The astrometric instrument comprises the two telescopes(Sect. 3.3.1), a dedicated area of 7 + 7 CCDs in the focal planedevoted to the sky mappers of the preceding and following tele-scope, and a dedicated area of 62 CCDs in the focal planewhere the two fields of view are combined onto the astromet-ric field (AF). The wavelength coverage of the astrometric in-strument, defining the unfiltered, white-light photometric G band(for Gaia), is 330–1050 nm (Carrasco et al. 2016; van Leeuwenet al. 2016). These photometric data have a high signal-to-noiseratio and are particularly suitable for variability studies (Eyeret al. 2016).

Unlike its predecessor mission Hipparcos, which selected itstargets for observation based on a predefined input catalogueloaded on board (Turon et al. 1993), Gaia performs an unbi-ased, flux-limited survey of the sky. This difference is primarilymotivated by the fact that an all-sky input catalogue at the spa-tial resolution of Gaia that is complete down to 20th mag, doesnot exist. Hence, autonomous, onboard object detection has beenimplemented through the Sky Mapper (Sect. 3.3.8), with the ad-vantage that transient sources such as supernovae and near-Earthasteroids are observed too. Every object crossing the focal planeis first detected either by SM strip 1 (SM1) or SM strip 2 (SM2).

These CCDs exclusively record, respectively, the objects fromthe preceding or from the following telescope. This is achievedthrough a physical mask that is placed in each telescope interme-diate image, at the M4/M4’ beam-combiner level (Sect. 3.3.1).

The SM CCDs are read out in full-frame TDI mode, whichmeans without windowing. Read samples, however, have a re-duced spatial resolution with an on-chip binning of 2 pixelsalong-scan × 2 pixels across-scan per sample. Windows are as-signed to detected objects and transmitted to ground; they mea-sure 40 × 6 samples of 2 × 2 pixels each for stars brighter thanG = 13 mag and 20 × 3 samples of 4 × 4 pixels each for fainterobjects. The SM CCD has the longest TDI gate, with 2900 TDIlines (2.85 s) effective integration time, permanently active to re-duce image degradation caused by optical distortions (which aresignificant at the edge of the field of view), and to reduce theCCD effective area susceptible to false detections generated bycosmic rays and solar protons.

The astrometric data acquired in the 62 CCDs in the AF fieldare binned on-chip in the across-scan direction over 12 pixels,except in the first AF strip (AF1) and for stars brighter than13 mag. For these stars, unbinned, single-pixel-resolution win-dows are often used in combination with temporary TDI-gate ac-tivation, during the period of time that corresponds to the bright-star window length, to shorten the CCD integration time andavoid pixel-level saturation. In AF1, across-scan information ismaintained at the CCD readout, but later binned by the onboardsoftware before transmission to ground; this permits the mea-suring of the actual velocities of objects through the focal planeto feed the attitude and control subsystem, to allow along- andacross-scan window propagation through the focal plane, and toidentify suspected moving objects, which receive a special, ad-ditional window either right on top or right below the nominalwindow in the photometric instrument (Sect. 3.3.6). The AF1data are also used on board for confirming the presence of de-tected objects. The along-scan window size in AF is 18 pixelsfor stars that are brighter than 16 mag and 12 pixels for fainterobjects. The astrometric instrument can handle object densitiesup to 1 050 000 objects deg−2 (Sect. 8.4). In denser areas, onlythe brightest stars are observed.

3.3.6. Photometric instrument

The photometric instrument measures the spectral energy distri-bution (SED) of all detected objects at the same angular resolu-tion and at the same epoch as the astrometric observations. Thisserves two goals:

1. The instrument provides astrophysical information for all ob-jects (Bailer-Jones et al. 2013), in particular astrophysicalclassification (for instance object type such as star, quasar,etc.) and astrophysical characterisation (for instance inter-stellar reddenings, surface gravities, metallicities, and effec-tive temperatures for stars, photometric redshifts for quasars,etc.).

2. The instrument enables chromatic corrections of the astro-metric centroid data induced by optical aberrations of thetelescope (Sect. 3.3.1).

Like the spectroscopic instrument (Sect. 3.3.7), the photo-metric instrument is highly integrated with the astrometric in-strument, using the same telescopes, the same focal plane (al-beit using a dedicated section of it), and the same sky-mapper(and AF1) function for object detection (and confirmation). Thephotometry function is achieved through two fused-silica prismsdispersing light entering the fields of view. One disperser, called



BP for blue photometer, operates in the wavelength range 330—680 nm; the other disperser, called RP for red photometer, cov-ers the wavelength range 640—1050 nm. Sometimes, BP andRP are collectively referred to as XP. Optical coatings depositedon the prisms, together with the telescope transmission and de-tector quantum efficiency, define the bandpasses. The prisms arelocated in the common path of the two telescopes, and mountedon the CCD cold radiator, directly in front of the focal plane.Both photometers are equipped with a dedicated strip of sevenCCDs each, which cover the full astrometric field of view in theacross-scan direction (see Sect. 3.3.2 for details on the photo-metric CCDs). This implies that the photometers see the same(number of) transits as the astrometric instrument.

The prisms disperse object images along the scan direc-tion and spread them over ∼45 pixels (for 15-mag objects):the along-scan window size is chosen as 60 pixels to allow forbackground subtraction (and window-propagation and window-placement quantisation errors). The spectral dispersion, whichmatches the earlier photometric-filter design described in Jordiet al. (2006), results from the natural dispersion curve of fusedsilica and varies in BP from 3 to 27 nm pixel−1 over the wave-length range 330—680 nm and in RP from 7 to 15 nm pixel−1over the wavelength range 640—1050 nm. The 76% energy ex-tent of the along-scan line-spread function varies along the BPspectrum from 1.3 pixels at 330 nm to 1.9 pixels at 680 nm andalong the RP spectrum from 3.5 pixels at 640 nm to 4.1 pixels at1050 nm.

For the majority of objects, BP and RP spectra are binnedon-chip in the across-scan direction over 12 pixels to formone-dimensional, along-scan spectra. Unbinned, single-pixel-resolution windows (of size 60 × 12 pixels2) are only used forstars brighter than G = 11.5 mag; this is often in combina-tion with temporary TDI-gate activation, during the period oftime corresponding to the bright-star window length, to shortenthe CCD integration time and avoid pixel-level saturation. Theobject-handling capability of the photometric instrument is lim-ited to 750 000 objects deg−2 (Sect. 8.4); only the brightest ob-jects receive a window in areas exceeding this density. Thedata quality, however, is already affected at lower densities bycontamination from the point spread function wings of nearbysources falling outside the window (degrading flux and back-ground estimation) and by blending with sources falling insidethe window (leading to window truncation and necessitating adeblending procedure; Busso et al. 2012).

3.3.7. Spectroscopic instrument

The spectroscopic instrument, known as the radial-velocity spec-trometer (RVS), obtains spectra of the bright end of the Gaiasample to provide:

1. radial velocities through Doppler-shift measurements usingcross-correlation for stars brighter than GRVS ≈ 16 mag(Sect. 8.4; David et al. 2014), which are required for kine-matical and dynamical studies of the Galactic populationsand for deriving good astrometry of nearby, fast-movingsources which show perspective acceleration (e.g. de Brui-jne & Eilers 2012);

2. coarse stellar parametrisation for stars brighter than GRVS ≈14.5 mag (e.g. Recio-Blanco et al. 2016);

3. astrophysical information, such as interstellar reddening, at-mospheric parameters, and rotational velocities, for starsbrighter than GRVS ≈ 12.5 mag (e.g. Recio-Blanco et al.2016);

4. individual element abundances for some elements (e.g. Fe,Ca, Mg, Ti, and Si) for stars brighter than GRVS ≈ 11 mag(e.g. Recio-Blanco et al. 2016),

where GRVS denotes the integrated, instrumental magnitude inthe spectroscopic bandpass (defined below).

The spectroscopic instrument (Cropper & Katz 2011), likethe photometric instrument (Sect. 3.3.6), is highly integratedwith the astrometric instrument, using the same telescopes, thesame focal plane (using a dedicated section of it), and the samesky-mapper (and AF1) function for object detection (and confir-mation). The actual (faint-end) selection of an object for RVS,however, is based on an onboard estimate of GRVS that is gen-erally derived from the RP spectrum collected just before theobject enters RVS. The RVS is an integral-field spectrographand the spectral dispersion of objects in the fields of view ismaterialised through an optical module with unit magnification,which is mounted in the common path of the two telescopesbetween the last telescope mirror (M6) and the focal plane.This module contains a blazed-transmission grating plate (usedin transmission in order +1), four fused-silica prismatic lenses(two with flat surfaces and two with spherical surfaces), and amultilayer-interference bandpass-filter plate to limit the wave-length range to 845–872 nm. This range was selected to cover theCa ii triplet, which is suitable for radial-velocity determinationover a wide range of metallicities, signal-to-noise ratios, tem-peratures, and luminosity classes in particular for abundant FGKstars, and which is also a well-known metallicity indicator andstellar parametriser (e.g. Terlevich et al. 1989; Kordopatis et al.2011). For early-type stars, the RVS wavelength range coversthe hydrogen Paschen series from which radial velocities can bederived. In addition, the wavelength range covers a diffuse inter-stellar band (DIB), located at 862 nm, which traces out interstel-lar reddening (e.g. Kučinskas & Vansevičius 2002; Munari et al.2008).

The dispersed light from the RVS illuminates a dedicatedarea of the focal plane containing 12 CCDs arranged in threestrips of four CCD rows (see Sect. 3.3.2 for details on the spec-troscopic CCDs). This implies that an object observed by RVShas 43% (1 − 4/7) fewer RVS focal plane transits than astro-metric and photometric focal plane transits. The grating platedisperses object images along the scan direction and spreadsthem over ∼1100 pixels (R = λ/∆λ ≈ 11 700, dispersion0.0245 nm pixel−1); the along-scan window size is 1296 pixelsto allow for background subtraction (and window-propagationand window-placement quantisation errors).

For the majority of objects, RVS spectra are binned on-chip in the across-scan direction over 10 pixels to formone-dimensional, along-scan spectra. The onboard software(Sect. 3.3.8) contains a provision to adapt this size to the in-stantaneous, straylight-dominated background level (Sect. 4.2),in view of optimising the signal-to-noise ratio of the spectra, butthis functionality is not being used. Single-pixel-resolution win-dows (of size 1296 × 10 pixels2) are only used for stars brighterthan GRVS = 7 mag. The object-handling capability of RVS islimited to 35 000 objects deg−2 (Sect. 8.4); in areas exceedingthis density, only the brightest objects receive a window. As forthe photometers, however, the data quality will be severely com-promised in dense areas by contamination from and blendingwith nearby sources.



3.3.8. Video processing unit and algorithms

Each CCD row in the focal plane (Sect. 3.3.2) is connected toits own video processing unit (VPU), essentially a computerin charge of commanding the CCDs and collecting the sciencedata and transmitting it to the onboard storage (Sect. 3.3.9). TheVPUs run the video processing algorithms (VPAs; Provost et al.2007), which are a collection of software routines configurablethrough a set of parameters that can be changed by telecom-mand. The seven VPUs are fully independent although each oneruns the same set of VPAs albeit (possibly) with different pa-rameter sets. Parameter updates are possible but require a tran-sition from VPU operational mode to VPU service mode, whichmeans a loss of science data of a few dozen seconds. The VPUsand VPAs have a large number of functions such as CCD com-mand generation, including deriving the TDI-line signals fromthe spacecraft master clock (Sect. 3.3.10) for the synchronisationof the CCD sequencing. The CCD TDI (line) period is defined as19,656 master-clock cycles and hence lasts 982.8 µs. The VPAsare also responsible for the detection, selection, and confirmationof objects. The detection algorithm uses full-frame SM data todiscriminate stars from spurious objects, such as cosmic rays andsolar protons, autonomously using PSF-based criteria; the pa-rameter settings adopted for operations guarantee a high level ofcompleteness down to the faint limit at G = 20.7 mag (Sect. 8.4)at the expense of spurious detections in the (diffraction) wingsof bright stars essentially passing unfiltered (de Bruijne et al.2015, ; in May 2016, a new set of parameters was uploadedthat accepts fewer false detections at the expense of a reduceddetection efficiency of objects beyond 20 mag). After detectionin SM, (the brightest) accepted objects are allocated a windowfrom the pool of available windows. A final confirmation of eachdetection is enabled by the CCD detectors in the first AF strip(AF1); this step eliminates false detections in SM caused by cos-mic rays or solar protons. Whether a detected object is actuallyselected or not for observation, i.e. receives a window, dependson a number of factors. Several limitations exist, for example indense areas or when multiple bright stars, each requiring single-pixel-resolution windows, are present in the same TDI line(s);in particular this is caused by the fact that the total number ofsamples in the serial register that Gaia can observe simultane-ously per CCD is 20 in AF, 71 in BP and RP, and 72 in RVS(Sect. 3.3.2). In case of a shortage of windows, object selection(or resource allocation, where resource refers to serial samples)is based on object priority; the latter is a user-defined attributewhich, in practice, is only a function of magnitude, where brightstars have higher priority. The VPAs assign windows based onthe onboard measured position and brightness of the object prop-agate windows through the focal plane, along-scan in line withthe spin rate and across-scan to follow the small, across-scan mo-tion of objects induced by the scanning law (Sect. 5.2). The win-dow management, meaning the collection of CCD sample data,the truncation of samples in case windows of nearby sources(partially) overlap, and packetisation and lossless compressionof the science data is also driven by the VPAs. In addition, theVPAs feed the (closed) attitude control loop with rate measure-ments based on the measured transit velocities of 13–18-magobjects between SM and AF1 (Sect. 3.2.1). They also govern theactivation of TDI gates for the along-scan duration of bright-star windows in AF, BP, and RP, and the periodic activationof charge-injection lines in AF, BP, and RP (Sect. 3.3.2). TheVPAs collect health and housekeeping data, such as pre-scandata for CCD-bias monitoring, detection-confirmation-selectionstatistics, object logs to enable CCD-readout reconstruction for

PEM non-uniformity calibration (Sect. 3.3.2), etc., collect BAMand WFS data (Sects. 3.3.4 and 3.3.3), and collect service-interface-function (SIF) data. The SIF function provides directaccess to the synchronous dynamic random-access memory ofthe VPU, allowing mo

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LJMU Research Onlineresearchonline.ljmu.ac.uk/5303/1/1609.04153v1.pdf · Astronomy & Astrophysics manuscript no. 29272 c ESO 2016 September 15, 2016 The Gaia mission Gaia Collaboration,